This should close #156

Because raising errors when unable to process
stuff instead of an exception is dumb.

This should close #151

Warn when there are encrypted fonts in an epub file

This should improve epub support

This is now required by ffmpeg

This should increase a bit the compability with Excel files

This commit takes care of removing residual metadata
added by mat2 during the cleaning of pdf.

Mypy doesn't like some annotation in web.py,
this commits aims at pleasing it.

Mounting new, empty filesystem on /tmp makes impossible to use mat2 for manipulating files stored there. Especially it breaks running tests while creating package and using /tmp as temporary builddir which is common setup in Arch Linux:
https://aur.archlinux.org/packages/mat2/#comment-721221